Enabling Thickness Uniformity and Control for Curved Optics

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One of the most important requirements for precision optics is thickness uniformity of thin film coatings. Achieving a high degree of uniformity on flat optics usually requires complex motion or restricted deposition zoned, which often lowers deposition rates. For curved optics, there is no practical way to achieve the uniformity needed through a conventional thin film deposition method.

A novel approach could help improve repeatability and uniformity for curved optics, however, ensuring that thin film requirements are met and opening the door for new applications in precision optics.

Precision Optics Coating Requirements

Most precision optics applications require the following characteristics and abilities when it comes to thin film deposition:

  • Multilayer stacks: Multilayer optical stacks are needed to create many coatings for precision optical coatings, such as optical filters. You need a thin film deposition method that can provide repeatability and uniformity for multilayer coatings on curved optics – which to this point is not achievable through conventional physical vapor deposition methods.
  • Repeatability: In precision optics, precision is a must to ensure performance. Coatings for optics must always hit within the desired spec, with very little room for error. A highly repeatable process ensures that you’ll have tight control from run to run, so you are always hitting the required specs with very little error.
  • Excellent thickness uniformity: Along with repeatability, uniformity across the coating is also needed. If there is any variation in thickness across the surface of the optic, the refractive index will also vary, which ultimately compromises performance. If one area of the optic meets the specs needed for an anti-reflective coating, but another area does not, it will compromise the entire coating and device yield will be lower.

How Inverted Cylindrical Magnetron Sputtering Helps

Planar magnetron sputtering, using cathodes in a planar configuration, or e-beam evaporation are generally acceptable methods for coating flat optics and achieving a high degree of precision and uniformity. When excellent precision is needed, ion beam sputtering is another option.

However, these methods typically feature slower deposition rates, and require process steps like masking or substrate motion to ensure thickness uniformity across the substrate. And none of these methods present a feasible solution for coating curved optics with uniform thin films. To do that, you need inverted cylindrical magnetron (ICM) sputtering.

ICM sputtering features an inverted cylindrical cathode, a unique configuration where the substrate is placed inside of the cathode. The sputtering direction is inward, as opposed to up or down. The target is a ring on the inside of the cathode, and because of its cylindrical shape, it allows the entire surface of the optic to be coated at the same time.

This configuration has major implications for curved optics. With ICM sputtering, it’s now possible to coat curved surfaces with excellent precision and thickness uniformity, including in multilayer stacks. This method ensures that the refractive index is the same across the entire optic, so you get the same performance. Potential applications for this configuration include single wear-resistant coatings, AR/HR coatings and optical filters on highly curved optics.

ICM sputtering can also greatly improve the thin film coating process for flat optics. Compared to other PVD methods, ICM offers the highest productivity and best process efficiency. You don’t need to use any masking, so more of the deposition plume is productive. The cylindrical cathode offers inherent uniformity, and you also have the option to implement some motion for even tighter control over thickness.

Invest in a Fully Integrated Solution

To realize all of the benefits of an inverted cylindrical cathode for curved or flat precision optics, you need to invest in a fully integrated solution that’s designed to achieve repeatability and uniformity. Denton Vacuum has the expertise and process knowledge needed to accomplish this. We have tested and developed a full ICM sputtering system design that achieves excellent precision for both curved and flat optics in real-world applications.

Our design offers higher deposition rates and better process efficiency than what is currently achievable through other methods for flat optics – and it opens the door for brand-new applications for curved optics, which cannot be uniformly coated through any other method. With our extensive knowledge and application-specific approach, we can also ensure your system is fully integrated with other sources and controls, to enable the best thin film performance.

To learn more about how an inverted cylindrical cathode can be integrated with your thin film deposition process, contact us.